Graphene is a 2-dimensional, semi-metallic, atomically-thin film in which carbon atoms are arranged into a sp2 honeycomb lattice structurally relying on in-plane, covalent σ-bonds. It has been successfully isolated for the first time in 2004 at the University of Manchester by A. K. Geim, K. S. Novoselov and his co-workers, by micromechanically exfoliating graphite crystals on top of Si/SiO2 film stacks. Significant breakthroughs in graphene applied research have been achieved: switching behaviors with fT in excess of hundreds GHz, high frequency photodetectors for high speed (10 Gb/s) optical communications, single molecule detectors, and high mobility transistors (˜105 cm2V−1s−). It is also chemically stable in non-oxidizing environments, and is mechanically very stiff. Its electronic transport properties have been found to be largely superior to those of materials traditionally employed in microelectronics. Therefore, graphene is one of the most promising candidates as material for post-CMOS applications.
Single layer graphene (SLG) is a bandgap-free semimetal. As a consequence, transistors using SLG as active channel exhibit a poor Ion/Ioff (generally ˜10), and cannot be switched off. This is one of the main limiting factors hampering the use of graphene in microelectronics for logic applications. To solve this problem, a bandgap would be required in SLG. A number of approaches have been pursued to induce bandgap opening in SLG; for example, when SLG is tailored in few nm-wide ribbons, a quantum confinement-induced bandgap appears (K. Nakada et al., Phys. Rev. B 54, 17954 (1996)).
A radically different approach can be pursued by using bilayer graphene (BLG) (E. McCann, Phys Rev B 74, (2006)). BLG—which consists of two SLG stacked vertically and interacting via their pi-bonds—shares with SLG the zero bandgap character and is therefore a semimetal too (see FIG. 1(b) (left)). However, a bandgap can be introduced in BLG if the inversion symmetry of the two stacked layers is broken by the use of an external electric field, applied perpendicularly to the BLG plane (see FIG. 1(b) (right). BLG then becomes a semiconductor with a bandgap that depends on the strength of the applied electric field. The maximum value of the bandgap that can be induced depends solely on interlayer coupling energy. So far, the bandgap opening in BLG by vertical symmetry breaking, as described above, has been achieved in two main flavors:
(i) Electric Displacement Field Generated by a Gate Electrode:
In this method, an external gate stack in direct contact with BLG (e.g. a top gate stack) is used to establish an electric displacement field perpendicular to the BLG plane. The field induces two different excess charge densities on the two layers of BLG, thus inducing a charge density asymmetry between the two layers (E. McCann, Phys Rev B 74, (2006)). The Coulomb interaction between the two asymmetric charges causes the opening of a bandgap between the conductance and valence energy bands in the BLG band diagram. An optical bandgap of 250 meV has been measured by infrared spectroscopy (F. Wang et al., Nature 459, 820 (2009)). A bandgap of at least 300 meV would however be needed to achieve an Ion/Ioff in excess of 104 (K. Majumdar, et al., 2010 International Electron Devices Meeting—Technical Digest, (2010)) required to allow for efficient switching of BLG-based transistors at room temperature. In order to achieve a 300 meV bandgap, the applied electric field has to be larger than 3.5 V/nm. On the other hand, when one applies such a high electric field across BLG, the charge density in BLG exceeds 1013 cm−2. Therefore, to switch off the transistor, the primary gate must generate a very high electric displacement field to compensate the excess charge and put the Fermi level into the bandgap which may reach the break-down voltage of the SiO2 dielectric.
(II) Opening Up of a Bandgap in BLG by Adsorbates:
In this method, the top layer of BLG is doped by the physisorption of atoms or molecules. Being a strictly 2D material, graphene is extremely sensitive to adsorbates and other molecules in direct contact with its surface. This property can be exploited to tailor the electronic properties of graphene. Literature reports on various examples of adsorbed species, ranging from metals and adatoms (J. H. Chen et al. Nat Phys 4, 377 (2008)), to organic compounds (C. Coletti et al., Phys Rev B 81, (2010)), inorganic salts (D. B. Farmer et al., Appl Phys Lett 94, (2009)), and gases (A. Ghosh, J. Exp. Nanosci. 4, 313 (2009)). Each of these species can provide either n- or p-type doping in graphene, depending on the difference in the electronegativity between graphene and the adsorbate. When considering BLG, an effective electric field can be induced by placing excess charge on the top layer, resulting in charge redistribution and asymmetry between top and bottom layers. Doping BLG by chemical physisorption resembles the effect of an external gating. So far, the opening of a bandgap in BLG via physisorption has been performed by using metal adatoms deposited on top of BLG, such as potassium and aluminum (B. N. Szafranek et al., Nano Lett 11, 2640 (2011)), by evaporation of organic molecules (C. Coletti, et al., Phys Rev B 81, (2010), and also by doping with oxygen or even moisture C.-T. L. Wenjing Zhang et al., Acs Nano 5, 7517 (2011)). However, the doping approaches listed above are not easily controlled as the dopants tend to spread non-uniformly on the BLG. Furthermore, the dopant tends to migrate and interact with graphene, creating defects. This migration also tends to favor the formation of aggregates over time, leading to device performance stability problems. Also, the deposition of dopants is so far hardly compatible with typical CMOS process flows.